The present invention relates to a method for fabricating a capacitor device including a capacitor dielectric film made from a ferroelectric film, and more particularly, it relates to a technique to prevent characteristic degradation of a ferroelectric film.
In a conventional fabrication method for a capacitor device including a capacitor dielectric film made from a ferroelectric film (hereinafter simply referred to as the ferroelectric memory), a lower electrode, the capacitor dielectric film made from the ferroelectric film and an upper electrode are successively formed on a semiconductor substrate, and the ferroelectric film is generally crystallized in the middle of the process for forming these elements.
A first conventional method for fabricating a ferroelectric memory will now be described with reference to FIGS. 7A through 7D.
First, as shown in FIG. 7A, on a semiconductor substrate 1 in which a pair of impurity diffusion layers (not shown) serving as a source region and a drain region have been formed, an insulting film 2 is formed, and then, a contact plug 3 in contact with one of the pair of impurity diffusion layers is formed in the insulating film 2. Thereafter, a first conducting film 4 that is made from a multilayer film of, for example, a Pt film and an IrOx film (oxygen barrier layer) and is used for forming a lower electrode, a ferroelectric film 5, and a second conducting film 6 of, for example, a Pt film are successively deposited on the insulating film 2.
Next, first annealing is performed in an oxygen atmosphere, so as to crystallize the ferroelectric film 5, and thus, a (crystallized) ferroelectric film 5A having a perovskite crystal structure is formed as shown in FIG. 7B.
Then, the second conducting film 6, the crystallized ferroelectric film 5A and the first conducting film 4 are successively patterned, thereby forming an upper electrode 6A from the second conducting film 6, a capacitor dielectric film 5B from the crystallized ferroelectric film 5A and a lower electrode 4A from the first conducting film 4 as shown in FIG. 7C.
Subsequently, second annealing is performed in an oxygen atmosphere, so that the crystal structure of the capacitor dielectric film 5B made from the crystallized ferroelectric film 5A, which has been damaged through plasma etching employed for the patterning, can be recovered.
Next, an interlayer insulating film 7 is deposited so as to cover a capacitor device including the upper electrode 6A, the capacitor dielectric film 5B and the lower electrode 4A as shown in FIG. 7D.
In the first conventional method, the perovskite crystal is grown from the faces of the ferroelectric film 5 respectively in contact with the first conducting film 4 and the second conducting film 6 through the first annealing performed in an oxygen atmosphere.
However, the plasma etching employed for patterning the second conducting film 6, the crystallized ferroelectric film 5A and the first conducting film 4 damages the capacitor dielectric film 5B made from the crystallized ferroelectric film 5A.
Therefore, the second annealing is performed in an oxygen atmosphere, so as to recover the damaged crystal structure of the capacitor dielectric film 5B.
A second conventional method for fabricating a ferroelectric memory disclosed in, for example, Japanese Laid-Open Patent Publication No. 11-297946 will now be described with reference to FIGS. 8A through 8D.
First, as shown in FIG. 8A, on a semiconductor substrate 1 in which a pair of impurity diffusion layers (not shown) serving as a source region and a drain region have been formed, an insulating film 2 is formed, and then, a contact plug 3 in contact with one of the pair of impurity diffusion layers is formed in the insulating film 2. Thereafter, a first conducting film 4 that is made from a multilayer film of, for example, a Pt film and an IrOx film (oxygen barrier layer) and is used for forming a lower electrode, a ferroelectric film 5 and a second conducting film 6 of, for example, a Pt film are successively deposited on the insulating film 2.
Next, the second conducting film 6, the ferroelectric film 5 and the first conducting film 4 are successively patterned, thereby forming an upper electrode 6A from the second conducting film 6, a patterned ferroelectric film 5C and a lower electrode 4A from the first conducting film 4 as shown in FIG. 8B.
Then, annealing is performed in an oxygen atmosphere, so as to crystallize the patterned ferroelectric film 5C. Thus, a (crystallized) ferroelectric film 5D having a perovskite crystal structure is formed as shown in FIG. 8C.
Subsequently, an interlayer insulating film 7 is deposited so as to cover a capacitor device including the upper electrode 6, the crystallized ferroelectric film 5D and the lower electrode 4 as shown in FIG. 8D.
In the second conventional method, after patterning the second conducting film 6, the ferroelectric film 5 and the first conducting film 4, the annealing is performed in an oxygen atmosphere so as to crystallize the patterned ferroelectric film 5C. Therefore, the crystallized ferroelectric film 5D can be prevented from being damaged by the plasma etching differently from the first conventional method, and hence, the resultant ferroelectric memory can attain a good characteristic.
However, when the capacitor device fabricated by the first or second conventional method is allowed to stand, a problem of characteristic degradation of the ferroelectric film occurs with the elapse of time. Specifically, an imprint phenomenon in which the polarization of the ferroelectric film is not inverted can occur, or the polarization of the ferroelectric film can be lowered.
Therefore, the reason why the characteristic of the ferroelectric film is degraded when the capacitor device is allowed stand has been variously examined. As a result, it has been found that the characteristic of the ferroelectric film is degraded because atmospheric moisture invades the ferroelectric film while the capacitor device is being left.
Also, it has been found that the characteristic of the ferroelectric film is degraded because the capacitor dielectric film is damaged in depositing an interlayer insulating film for covering the capacitor device.